Radial Engines
 

Why do radial engines have an odd number of cylinders -- 3, 5, 7, 9, 11? The only ones I know of that have an even number of cylinders have two rows of cylinders. Internally, is there one master con rod, with the other cylinders attached to bosses on the big end of the main con rod?

Yes,
called a master rod

Even the R 2800 is 2 rows of 9 making 18. Funny I still remember the firing order. Plus 11 minus 7. In other words if you can't plus 11 you minus 7 and vice versa ie 1, 12, 5 etc

"Why do radial engines have an odd number of cylinders "

I remember seeing an explanation of that, years and years ago. Has to do with the forces applied when cylinders fire.

Les

It's really quite simple when you think about it. You're talking about 4-stroke engines, right ? So the engine has to go through two complete revolutions to fire all cylinders.

Draw yourself a front view of a radial engine. Make it a 5 cylinder for simplicity. The engine fires on alternating cylinders, going around the circle. If you fire 1,2,3,4,5, 1,2,3,4,5, 1,2,3,4,5 etc, you would be firing every cylinder on every revolution. But 4-strokes need to fire each cylinder only once for every two revolutions. So you fire one cylinder (starting with #1), then skip #2, fire #3, skip4, fire #5. Now you've made the first revolution. Beginning the second revolution, you skip #1 (already fired last time), fire #2, skip #3 and fire #4. Two complete revolutions and all cylinders fired !

If you try to fire every other cylinder with an even number of cylinders, you are forced to fire two adjacent cylinders at some point. Draw yourself a 4 cylinder radial and see what happens. Fire #1, skip #2, fire #3, skip #4, fire #1, skip #2, fire #3, skip #4 ...... oops, I never managed to fire 2 and 4 ....... So an even number of cylinders won't work.

Why the even number of cylinders on a two-row radial ? Well, a two-row is just that -- two radial engines one behind the other running on a common crankshaft. If you multiply an odd number by two, you always get an even number. (Definition of an "even number": A number that can be divided by two.)

Hope this helps,

Dick

Thanks, Dick. That makes sense. So, with a 5-cylinder radial you would get 3 power strokes on one rotation and 2 on the next.

Glad it makes sense. I didn't want to make things too complicated, but here is something else to think about: Every piston makes a full up/down stroke with every revolution. When #1 is at the top and firing, #2 is just finishing its exhaust stroke and ready to start down on its intake stroke. That's why you can have just one crank throw for all the cylinders. The valve timing (via the cam) is what decides which cylinders are ready to fire and which ones are still getting rid of the burned gasses from the prior firing.

It's all rather ingenious, especially considering the whole thing had been worked out by 1900.

Dick

Hey, back then we came up with ingenious and useful mechanisms, now we come up with ingenious ways to waste time doing nothing (e.g. Facebook, Twitter, video games...)

Laser pointers and cats - and we still get patents for these "ideas".

And posting on RCU, of course

I don't know about you but I found this very educational! I've been pondering this question for a while!

If you want to stretch your pondering a bit further consider the camshaft. Unlike a automobile engine where each cam lobe drives only one valve, in the radial all the intake valves are driven by the intake cam and all the exhaust valves are driven by the exhaust cam. This really does require an odd number of cylinders!
Chuck

If they had a even count on firing you would never have "THAT" sweet sound.. :cool:

Its all for the balance of the engine.. if it was even, the way it would have to fire would make the engine vibrate so much you would prob loose a engine at take off.. There is some very good reading about the operation of them...

While on Radial Engines, it has always amazes me that many of the WW-I full scale fighter airplanes had radial engines where the prop was bolted to the engine, the whole engine rotated, and the crankshaft was stationary. I believe there was no throttle control to the engine; it only ran at full throttle. To achieve a lower speed, like during taxiing, the electric ignition was interrupted.

Yes, the "rotary" engines are pretty unique. I have the privilege of working on the crew to launch and recover full scale WWI airplanes at a large private collection when demonstrations are flown each year. The sounds especially are different from any other engines I've ever heard.

Regarding throttling the rotary engines, not all are alike. There were several different manufacturers and engine sizes ranging from 50HP up to more than 200HP. The designs varied a lot. For throttling, some engines ran wide open unless the spark was interrupted, while others had primitive carburetors. The carburetor types typically had individual control of fuel and air, so the pilot had to carefully coordinate how much air and how much fuel he gave the engine, lest it quit lean or rich. On the ignition interruptor designs, some of the engines had a pilot controlled lever by which cylinders could be individually turned off, leaving the engine running on fewer cylinders. Imagine, if you will, a nine cylinder engine running on 7, or 5, or 3. Both the carburetor and the interruptor engines still had a "blip" switch on the stick where all cylinders could be shut off a few seconds at a time for very low power operations.

There was a discussion of the rotaries on RCU about a month ago, including videos of a Tabloid, a Camel, and a Snipe. You can hear the different types of throttling take place in the videos. The Tabloid and Snipe use intake restriction (carburetor) type throttling, whereas the Camel uses ignition interruption. The odd, flat, popping noises from the Camel are the sounds of interrupted ignition.

http://www.rcuniverse.com/forum/rc-s...mel-snipe.html (Post #7 has the links to the videos)

Dick

That's very interesting stuff, Dick. I would love to see an RC rotary engine like that (but with a good carb, lol).